I contacted technical staff at KMSP and, at first, they didn't believe this was their problem. After seeing recorded file samples from various dates, and seeing that the files that don't play right were recorded after a point in time when some equipment in the station had been changed, and getting a second report of a mythtv playback problem with KMSP recordings, they are thinking they may have a problem.

Key details:
1. The playback problem with KMSP recordings started sometime between May 2010 and Sept 2010.
2. I have not changed ANY mythtv settings, files, updates or hardware on my KM box for OVER A YEAR, and certainly not over the summer of 2010.
3. I did not change my tv antenna or coax over the summer of 2010.
4. In fall 2010, I checked out the channel 9.1 8VSB signal using a spectrum analyzer to be sure the RF envelope from my antenna is pretty flat across the entire 6MHz VHF 9 tv channel - it is. I only live 15 miles from the KMSP antenna tower, and reception has never been a problem.
5. There is no problem with recordings off any of the 1920x1080i over-the-air tv channels. KMSP 9.1 is 1280x720p.
6. Myth recordings made fall 2010 off KMSP Fox 9.1 do NOT play in VLC Player. VLC Player stops on the first frame and just sits there.
7. Myth recordings made in May 2010 (or before) off KMSP Fox 9.1 play just fine in VLC Player.
8. If I move KMSP recordings into the VIDEO folder of Knoppmyth (which is configured to use mplayer for playback) the files play smoothly.
9. I tried several different xorg output resolutions (edited the xorg.conf file) to other values (1920x1080, 1280x720). The stuttering occurs at all the screen resolutions I tried.
10. The R5.5 KM box at my friend's house shows the same symptoms - ruling out a hardware problem specific to my R5.5 KM computer.
10. over the summer of 2010 (late August?), Fox updated KMSP's "Fox Splicer" box to the new version 2.0

OMG that's the problem that I've been having with my Fox shows 1080 720 doesn't matter Family Guy is especially bad. I thought It was a hardware problem with my ancient gear. I too am getting Fox OTA. Sometimes it happens on other shows but it is defenitly worst on Fox. Maybe it's some kind of copy protection? (which is ironic because nothing makes me want to torrent like a crappy recording)

neutron68:Are you using XVMC assisted decoding? Your sig looks like you have enough horse power to go without it. Try turning XVMC off if you are using it.

Hi. No, I'm not using XVMC. This install of R5.5 has always run with the STANDARD decoder. With the dual core processor, I have smooth OSD fadeaways without resorting to XMVC. For the first 1.5 years of this R5.5 install, channel 9.1 was fine, then over the summer, WHAM.....stutter stutter!
Could this be the broadcast flag in action?

I downgraded a 6 machine temporarily to 5.5 because I was having ir blaster problems. The result was a lot of frame dropping on HD...

(I eventually resolved the blaster problems with 6) and put it back on that machine.. the HD handling problems went away.. Just a thought...

I am considering R6, but I note above that mattbatt has R6 running and also has issues with a Fox over-the-air station stuttering! Changing to R6 will be one of the last steps in this troubleshooting exercise.

Right now, I'm going on the premise: R5.5 has worked fine for 1.5 years. I didn't change anything. Why did this stuttering start over the summer?

Have you looked at the system load when the stuttering is occurring versus when it isn't? I'd start by ssh-ing into the machine and watching top, and follow that up with iostat to watch the disk load. If you see a significant bump in CPU or disk bandwidth used that may point to the cause.

Are the recording files significantly bigger or smaller since the change? (I'd calculate an average Mb/minute rate to get a good number) This may indicate a different bit rate or extra data streams. If it's the later you can tweak your recording profile to cut out extraneous stuff that can make the decoder work harder.

Have they stretched or shortened the key frame interval? Longer intervals also tend to mean more work for the decoder.

Going on the assumption that KMSP and Fox changed their "digital formula", is it possible that some versions of mpeg decoder software respond differently? Is there a way to tell what version of the internal mythtv player is?

I got a report from a friend who's R5.5 KM box I sheppard, and he says he does not see stuttering with recordings from the same KMSP over-the-air signal. We have identical motherboards, same HD-3000 receiver cards, both are using the onboard nvidia 6150 chip for display and both have R5.5 on our machines.

The only differences I can think of between the 2 machines is versions of some software.
1. about a year ago, I updated the version of mplayer, and maybe ffmpeg software on my machine. I think his may still be stock versions of the mpeg software that comes with stock R5.5.
2. I think I updated the nvidia driver version on my machine about a year ago. I can't recall if I updated his, too.

Does the internal mythtv player use mplayer or ffmpeg? Would different versions of these 2 packages explain why the built-in myth player stutters on my KM machine but does not stutter on kent's KM machine?

It really shouldn't matter what version of ffmpeg you have installed because the internal MythTV player uses it own modified and embedded version of ffmpeg.

Do you see anything in the mythfronend.log or mythbackend.log? Can you playback the recording with mplayer and does it studder there? That might narrow down if it is a recording problem or playback problem.

It really shouldn't matter what version of ffmpeg you have installed because the internal MythTV player uses it own modified and embedded version of ffmpeg.

Do you see anything in the mythfronend.log or mythbackend.log? Can you playback the recording with mplayer and does it studder there? That might narrow down if it is a recording problem or playback problem.

Britney

Hi Britney. If you are still in SW MN, you may have KMSP on your cable system and could give it a look.

If I move KMSP recordings into the VIDEO folder of Knoppmyth (which is configured to use mplayer for playback) the files play smoothly.
Also, if I copy the files off to another machine and play them with mplayer, they play smoothly.

I just played back last night's Family Guy recording and the frontend log had a double framerate (deinterlace) error in it. I wonder if I have any logs from May 2010 still on the machine to see if that error was there during playback of KMSP recordings, back when they didn't stutter??
Do the logs get archived and put into a tar file somewhere? If yes, where?

I don't know how to tell if KMSP changed the key frame interval. Any suggested software or command? I believe that Fox 720p has always been a framerate of 60 frames per second.

"Key frames" are part of the MPEG encoding scheme, which starts with a key frame, which is a complete screen image, followed by a bunch of incremental changes to that frame for the subsequent frames, until finally another key frame comes along.

The maximum interval between keyframes is one of those things that can vary. The easiest way to check it out is to go into cut-point editing mode, advance to the next keyframe, note it's position, then step forward one more. Do that a few times and you should quickly have a ballpark figure for the key frame interval.

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